Bistable liquid crystal display device including improved addressing means

ABSTRACT

A method of electrically addressing a matrix screen of bistable nematic liquid crystals with breaking of anchoring is disclosed. Controlled electrical signals are applied respectively to row electrodes and to column electrodes of the screen. A plurality of rows are simultaneously addressed using similar row signals that are offset in time by a duration greater than or equal to the time column voltages that are applied. The row addressing signals have, in a first period, at least one voltage value serving to break the anchoring of all of the pixels in the row. This is followed by a second period that enables the final states of the pixels making up the address row to be determined. The final states are a function of the value of each of the electrical signals applied to the corresponding columns.

CROSS REFERENCE TO RELATED APPLICATIONS

The present application is the National Stage of InternationalApplication No. PCT/FR03/01240, filed on Apr. 17, 2003.

BACKGROUND OF THE INVENTION

The present invention relates to the field of liquid crystal displaydevices, and more precisely to a method and a device for controlling theswitching between two states of a multiplexed bistable nematic display.

Addressing a Passive Liquid Crystal Display of the STN Type

The abbreviation STN stands for “super twisted nematic”. It relates todisplays having super-twisted molecule structure.

The Principle of Multiplexing—and its Limitations

Passive screens capable of displaying a large number of rows (e.g. STNtechnology makes it possible to obtain up to about 500 rows) use anaddressing technique known as multiplexing.

With a matrix screen of medium resolution, the person skilled in the artknows that there is no question of individually connecting each pixel toan independent control electrode, since that would require oneconnection per pixel which is technologically impossible as soon as thescreen becomes complex. It is possible to save on connections by makinguse of the multiplexing technique when the electro-optical effect usedis non-linear, as applies to the usual liquid crystal techniques knownas twisted nematic (TN) and super twisted nematic (STN). Each pixel isconstituted by the intersection between a row electrode and a columnelectrode. The pixels are arranged in a matrix system with n groups eachhaving m pixels. For example, there are n rows and m columns for matrixscreens or n digits and m digit portions for digit displays. Insequential addressing mode, which is the mode that is in most widespreaduse, a single row is selected at a time. While a row is selected, columnsignals are applied simultaneously to all of the pixels in the row, andthen the technique moves on to the following row, and so on, down to thelast row. The frequency at which each row is refreshed electrically mustbe high enough to obtain good visual characteristics for the displayedimage (about 50 times per second).

The time required for addressing the image is equal to the time requiredfor addressing one row multiplied by the number of rows n. With thatmethod, a mere m+n connections suffice for addressing a screen of m×npixels, where m is the number of columns in the matrix in question. Amultiplexed matrix screen is shown in FIG. 1.

The signal to which the pixel is subjected is the difference between thesignal applied to the row and the signal applied to the column for whichthe pixel occupies the intersection.

The type of screen as shown in FIG. 1 is said to be a “passive screen”:it does not include active elements enabling the pixels to beelectrically isolated from one another. A row electrode is common to allof the pixels of the row and a column electrode is common to all of thepixels of the column, without there being any active element (e.g. atransistor). As a result, passive screens are much simpler tomanufacture than are active screens which include one transistor or onediode per pixel.

The drawback of multiplexing is that a pixel is addressed by columnsignals throughout the time the image is being addressed, and not onlywhile its own row is being activated. That is to say, while the image isbeing written, a pixel on the screen receives in succession the columnsignals for its entire column. It can be assumed that the signalsapplied to the pixel outside the time during which its row is selectedact as interfering signals that have an effect on the electro-opticalresponse of the liquid crystal pixel. More precisely, for passivematrices of the TN, STN, or similar type, the state of the liquidcrystal in a pixel depends almost exclusively only on the root meansquare (rms) value of the voltage which is applied thereto during theimage addressing time, under the usual operating conditions. As aresult, the final state of the liquid crystal molecules, which meansessentially the optical transmission state of the pixel, is determinedby the rms voltage applied during the image addressing time. Optimizingrow and column signals leads to the Alt and Plesko criterion (Alt, P.M., et al., IEEE Trans Electron Devices, ED 21: pp 146-155) which puts apractical limit on the number of rows a screen can have.

One of the principles limiting sequential addressing by one row at atime is that the voltage applied to a given pixel passes through a veryclearly marked maximum each time its row is selected. The liquid crystalof the pixel then presents an instantaneous response characterized byrelaxing between two occasions on which the row is addressed, i.e.between two consecutive frames. This leads to a high level of flickerand to an apparent loss of contrast. This effect is commonly referred toas “frame response”. To limit this effect, it is necessary to select aliquid crystal having a response time that is slow, to the detriment ofthe speed performance of the display.

Reduction of the “Frame Response” Effect by Multi-Line Addressing (MLA)

U.S. Pat. No. 5,420,604 proposes a novel addressing technique for an STNscreen characterized by selecting a plurality of rows simultaneously(referred to as MLA or MLS for multi-line selection). That methodrelates solely to passive screens in which the optical response of theliquid crystal is a function mainly of the applied rms voltage.

By addressing a plurality of rows simultaneously, it is possible toreduce the “frame response” effect considerably, since during the frametime, the row receives not only one, but a plurality of selectionpulses. It is then possible to use a liquid crystal having a fastresponse time.

Implementing MLA requires row selection signals to be generated that are“normalized and orthogonal”, and sometimes requires an image memory tobe incorporated in the screen driver circuit. That leads to controlelectronics of greater expense.

Reference can usefully be made to the above-mentioned document in orderto understand the kind of signals required. The term “normalized” meansthat the row selection signals must be normalized so that they allpossess the same rms value. The term “orthogonal” means that the rowselection signals must be adapted so that multiplying any one of the rowselection signals by the signal for a distinct row gives a signal inwhich the integral over the frame period is zero.

Addressing a Bistable LCD of the Cholesteric Type (Planar-Conical FocusTransition)

PCT Patent No. WO 00/74030 describes a method of addressing a pluralityof rows simultaneously applied to a screen using a bistable liquidcrystal having a chiral component (of the cholesteric type). In thatdocument, rows that are addressed simultaneously must be addressed bysignals that are mutually orthogonal. It is necessary to controlaccurately the rms voltage applied to the pixel during some of the fouraddressing stages of a screen based on a cholesteric liquidcrystal-based screen. The use of orthogonal signals for addressing therow enables the voltages to be controlled effectively.

Description of the Bistable Screen (FIG. 2)

Recently, a new bistable display has been proposed and is described inFrench Patent No. 96/04447.

It is constituted by a cholesteric or chiralized nematic liquid crystallayer between two plates or substrates, at least one of which istransparent. Two electrodes are disposed on the respective substratesand serve to apply electrical control signals to the chiralized nematicliquid crystal situated between them. On the electrodes, anchor layersorient the liquid crystal molecules in desired directions. On a masterplate, molecules are anchored strongly with a slight incline, while onthe slave plate, anchoring is weak and flat. The anchoring of themolecules to these surfaces is monostable.

The device also includes an optical system.

The two bistable textures U (uniform or weakly twisted) and T (twisted)of the liquid crystal are stable without an applied electric field. Thisis obtained for a zero or small angle between the anchor direction onthe master plate and on the slave plate. The twists of the two texturesdiffer in absolute value by about 180°. The spontaneous pitch p₀ of thenematic is selected to be close to four times the thickness d of thecell (p₀≈4·d) in order to ensure that the energies of the textures U andT are essentially equal. With no applied field, there exists no otherstate with lower energy: U and T are genuinely bistable.

Switching from One Texture to the Other by Breaking the Anchoring

Physical Principle

The two bistable textures are topologically distinct, and it is notpossible to transform one into the other by continuous volumedistortion. Transformation from texture U to texture T, or vice versa,therefore requires either anchoring on the surfaces to be broken, as isinduced by a strong external field, or else a disinclination line to bedisplaced. This second phenomenon which is much slower than the firstcan be ignored and is not described in detail below.

Any liquid crystal alignment layer can be characterized by zenithalanchoring energy Az. This energy is always finite. It can be seen shownthat there then exists a threshold field E_(c) that is also finite(threshold for breaking the anchoring), which gives a homeotropictexture (H) at the surface regardless of the preceding texture with noapplied field.

Breaking anchoring requires the application of a field that is not lessthan the threshold field E_(c). The field must be applied for asufficient length of time to ensure that the reorientation of the liquidcrystal in the vicinity of the surface leads to the homeotropic texture.This minimum length of time depends on the amplitude of the appliedfield, and also on the physical characteristics of the liquid crystaland of the alignment layer.

For the static situation (fields applied for a few milliseconds orlonger), $E_{c} \approx \frac{Az}{\sqrt{K_{33}ɛ_{0}\Delta\quad ɛ}}$where Az is the zenith anchoring energy of the surface, K₃₃ is theelastic bending coefficient of the liquid crystal, Δ∈ is its relativedielectric anisotropy, and ∈₀ is the dielectric constant of a vacuum.

Vc is defined as the voltage for breaking anchoring such that:Vc=E_(c)·d where d is the thickness of the liquid crystal cell.

The anchoring is said to be broken when the molecules are normal to theplate in the vicinity of said surface, and the return torque exerted bythe surface on the molecules is zero. In practice, it suffices for thedifference between the orientation of the molecules and the normal tothe surface to be sufficiently small, e.g. less than 0.5°, and for thetorque which is applied to the molecules at the surface to besufficiently small. When these conditions are united, the nematicmolecules close to the broken surface are in unstable equilibrium whenthe electric field is switched off, and can return either to theirinitial orientation, or else turn in the opposite direction so as toinduce a new texture differing from the initial texture by a twist of180°.

The final texture is determined by controlling the waveform of theapplied electrical signal, and in particular it depends on the way inwhich the field is returned to zero.

Lowering the voltage of the pulse progressively minimizes flow, withmolecules close to the master plate descending slowly towards theirequilibrium state, so that their elastic coupling with the molecules inthe center of the sample causes them to incline likewise in the samedirection, this movement diffusing to the slave plate where themolecules incline in turn quickly into the same direction, assisted bythe surface torque. The uniform state U then builds up progressively atthe center of the cell.

When the field drops suddenly, the orientation of the liquid crystal ismodified, initially at the vicinity of the strong surface (master plate)with a surface relaxation time equal to$\frac{\gamma_{i}L^{2}}{K},{{{where}\quad L} = \frac{K_{33}}{Az}}$is the extrapolation length of the strong layer and γ₁ is the viscosityin rotation of the liquid crystal. This time is typically of the orderof one-tenth of a microsecond (μs).

Switching the strong surface in such a short length of time leads to astrong flow close to said surface, which diffuses into the volume andreaches the weak surface (slave plate) after a characteristic length oftime that is shorter than one microsecond. The shear induced on the weaksurface (slave plate) creates a hydrodynamic torque on the molecules ofsaid surface. This torque is in the opposite direction to the elastictorque induced by the inclination of the master plate. When the shear isstrong enough, the hydrodynamic torque on the weak surface is thestronger torque, thereby promoting the twisted texture T. When the shearis weaker, the elastic torque on the weak surface is stronger, and itinduces the uniform texture U.

The direction of rotation of the molecules in the cell is represented byan arrow in FIG. 2.

Thereafter the volume reorients, with a characteristic volume relaxationtime τ_(vol) equal to $\frac{\gamma_{1}d^{2}}{K}$where d is the thickness of the cell. This time, which is typically ofmillisecond order, is much greater than the relaxation time of thestrong surface.Practical Embodiment

In general, the switching of a BiNem liquid crystal takes place in twostages:

First Stage: Stage of Breaking Anchoring, Written C

The stage C consists in applying an electrical signal characterized bythe fact that it breaks the anchoring on the slave plate. In general,the shorter the stage C, the greater the peak signal amplitude thatneeds to be applied.

For given amplitude and duration, the detailed waveform of the signal(slopes, intermediate levels, . . . ) does not have a determining effecton the behavior of the following stage, providing that anchoring isindeed broken.

Second Stage: Selection Stage, Written S

The voltage applied during the stage S must enable one of the twobistable textures U or T to be selected. Given the above-explainedeffect, it is the falling waveform of the electrical signal applied tothe terminals of each pixel that determine switchover from one textureto the other.

The term “writing” is used arbitrarily for switching to the twistedtexture T and the term “deleting” is used arbitrarily for switching tothe uniform texture U.

To write a pixel, i.e. to switch its texture to T, it is necessary:

Stage C: Breaking Anchoring

To apply a pulse delivering a field greater than the field for breakinganchoring on the slave plate and to wait for long enough for themolecules to rise in the pixel. The breaking field is a function of theelastic and electrical properties of the liquid crystal material and ofits interaction with the anchoring layer deposited on the slave plate ofthe cell. It can lie in the range a few volts to about 10 volts permicrometer (V/μm). The time required for the molecules to lift isproportional to the rotational viscosity γ and inversely proportional tothe dielectric anisotropy of the material used, and also to the squareof the applied field. In practice, this time can be lowered to a fewmicroseconds for fields of about 20 V/μm.

Stage S: Selecting the Texture

Thereafter it suffices to lower the field quickly, creating a suddendrop of the control voltage in a few microseconds or at most in a fewtens of microseconds. This sudden drop of voltage, of amplitude ΔV, issuch that it is capable of inducing a sufficiently intense hydrodynamiceffect in the liquid crystal. To produce the texture T, this drop mustnecessarily cause the applied voltage to switch from a value greaterthan the anchoring breaking voltage Vc to a value that is smaller thanthat. The time required for the applied field to drop is less thanone-tenth its duration or less than 50 microseconds with long pulses.

FIGS. 3 a 1 and 3 a 2 show two implementations of pulses that induce thetexture T.

In FIG. 3 a 1, the pulse comprises a first sequence of duration τ₁ ofamplitude P1 such that P1>Vc, followed by a second sequence of durationτ₂ of amplitude P2 slightly smaller than P1 such that P2>Vc and P2>ΔV,which second sequence switches suddenly to zero. In FIG. 3 a 2, thepulse comprises a first sequence of duration τ₁ of amplitude P1>Vcfollowed by a second sequence of duration τ₂ and of amplitude P2 suchthat P2<Vc and: P1−P2>ΔV.

Stage C: Breaking Anchoring

To delete it is necessary to apply a pulse supplying a field greaterthan the anchoring breaking field on the slave plate and to wait longenough to allow the molecules to lift in the pixel, as when writing.

Stage S: Selecting the Texture

French Patent No. 96/04447 proposes two ways of implementing a “slow”descent, as shown diagrammatically in FIGS. 3 b 1 and 3 b 2. The deletesignal is either a pulse of duration τ₁ and amplitude P1 followed by aslope of duration τ₂ with a descent time that is greater than threetimes the duration of the pulse (FIG. 3 b 1), or else a staircasedescent in the form of a signal having two plateaus (FIG. 3 b 2) (firstsequence of duration τ₁ and amplitude P1, followed by a second sequenceof duration τ₂ and amplitude P2 such that either P2>Vc and P2<ΔV, orelse P2<Vc and P1−P2<ΔV). The staircase descent with two steps is easierto implement with digital electronic means, so the slope is notdescribed in detail herein. Nevertheless, it is possible to imaginedevising a descent with a number of plateaus that is greater than two.

The waveforms of pulses characteristic of switching to one or the otherof the textures are given in FIG. 3 (refer to French Patent No. 96/04447and Giocondo, M., et al., “Write and erase mechanism of surfacecontrolled bistable nematic pixel,” Eur. Phys. J. AP., Vol. 5, pp227-230 (1999)). The durations and the voltages of the plateaus (P1, τ₁)and (P2, τ₂) have been determined experimentally for the examples givenbelow.

The Multiplexing Principle Applied to the Bistable Nematic Devices(BiNem)

The BiNem screens under consideration are likewise in the form of n×mpixels (FIG. 1) each pixel being located at the intersection of twoperpendicular conducive strips disposed on the two respective substratesas described above. The pixel of row N+1 and column M is shown shaded.The device has connections and electronic circuits placed on thesubstrate or on auxiliary cards.

The writing and deleting signals applied to the pixels are made bycombining row signals and column signals. They enable the screens inquestion to be written and deleted row by row, i.e. quickly.

Signals must be applied to the rows and the columns such that thevoltages that result across the terminals of a pixel are of a type shownin FIG. 3: the voltage applied to the pixel during the row write timemust be equal to a pulse which, on request, comes to an end eithersuddenly, leading to a sudden drop of voltage greater than or equal toΔV so as to create the twisted texture T (usually optically black), orelse to descend progressively in steps so as to create the uniformtexture U (the state which is usually optically bright).

The possibility of switching between the textures T and U and viceversa, by multiplexing, is demonstrated by the electro-optical curvegiven in FIG. 4: the BiNem pixel is addressed with a pulse having twoplateaus having a fixed value P1 and a variable value P2. Opticaltransmission is given as a function of the value of the second plateauP2, with P1=16 V. The pulse durations are 0.8 milliseconds (ms). Giventhe orientation of the polarizers in this example, a transmissionminimum corresponds to the state T and a transmission maximumcorresponds to the state U.

Writing Zones

For voltages P2 greater than about 11 volts, the voltage drop at the endof the plateau 2 is sufficient for writing. For voltages P2 less than 5volts, the voltage drop at the end of the time τ₁ has written, thevoltage of the plateau 2 is less than Vc, the voltage drop at its endcan no longer cause the texture to switch.

The value of the voltage drop ΔV needed for writing is equal to about 10volts, for P1=16 V and Vc=6 V.

Delete Zone

It can be seen from the curve in FIG. 4 that deleting takes place for avoltage P2E lying in the range 6 V to 9 V. In this voltage range, at theend of time τ₁, the molecules close to the slave plate are entrained bythe flow and thus in the write direction. During plateau 2, slightlyabove the breaking voltage, they become almost vertical while beingslightly inclined in the delete direction because of the elasticcoupling with the master plate. At the end of time τ₂, the voltage dropof less than ΔV is too small for the second flow to cause the moleculesto stand upright and fall in its direction, and thus write. The “slow”descent is thus implemented in two steps.

The values of the second plateau corresponding to one or other of thetextures are shown in FIG. 5.

In this example, during stage C of duration τ₁, a voltage P1 is appliedthat is suitable for breaking anchoring, and during the stage S ofduration τ₂, a voltage P2 is applied. The texture obtained depends onthe value of P2.

Multiplexing BiNems in the Prior Art

F1 and F2 are defined as two operating points situated at the rising orfalling point of inflection in the optical transmission curve of FIG. 4.We consider F2 by way of example. The voltage corresponding to the pointF2 is equal to 11 V, and may correspond to the value of the secondplateau A2 of the row signal. The value of the column voltage C=2 Vcorresponds to the voltage difference needed to obtain the pixel voltagecorresponding either to texture T (minimum transmission) or to texture U(maximum transmission). The value of the second plateau applied to thepixel is then either P2I=A2+C for writing (texture U) or else P2E=A2−Cfor deleting (texture T) with:

for the row signal: A1=16 V; A2=10 V;

for the column signal: C=2 V;

for the signal across the terminals of the pixel: P1=16 V; P2E=8 V;P2I=12 V.

These values vary depending on the properties of the liquid crystal andof the alignment layer, and can easily be adjusted for other screensmade on the same principles using other materials. An example is givenin Dozov, I., et al, “Recent improvements of bistable nematic displaysswitched by anchoring breaking,” Proceedings of SAID 2001, pp. 224-227(2001).

FIG. 6 shows the principle of row and column signals for writing anddeleting when above-defined operating point F2 has been selected. Therow signal (FIG. 6 a) comprises two plateaus: the first provides thevoltage A1 during τ₁, the second A2 during τ₂. The column signal (FIGS.6 b and 6 c) of amplitude C is applied solely during time τ₂, and ispositive or negative depending on whether it is desired to delete or towrite. The time τ₃ separates two row pulses. FIGS. 6 d and 6 e show thesignals applied respectively to the terminals of a deleted pixel and tothe terminals of a written pixel. These signals are very simple andenable all of their parameters to be adjusted easily to thecharacteristics of the screen.

Optimizing the Column Signal as Described in French Patent No. 02/1448

In a patent application filed in France on Feb. 6, 2002 under the No.02/01448, the Applicant has described various improvements to displaysof the BiNem type seeking to optimize the column signal. Thoseimprovements are recalled below in order to incorporate them in thepresent patent application.

In that document, the parameters of the signals applied to the columnelectrodes of the screen are adapted so as to reduce the rms voltage ofthe interfering pixel pulses to a value which is lower than theFreedericksz voltage, so as to reduce the interfering optical effects ofthe addressing.

EXAMPLE 1 Reducing the Duration of the Column Pulse

The new column signal C′ is applied for a time τ_(c)<τ₂, while keepingthe amplitude C′ at substantially the same order as C, since an increasein C′ would increase the rms value of the interfering voltage applied tothe pixels, and decreasing C′ would prevent switching from taking placebecause of the limitation shown by the electro-optical curve of FIG. 4.In this example, the selection stage is shortened compared with thepreceding circumstances by a duration τ_(c).

The signals corresponding to Example 1 are given in FIG. 7.

FIG. 7 shows in FIG. 7 a: a row signal, in FIG. 7 b: a column deletesignal, in FIG. 7 c: a column write signal, in FIG. 7 d: a delete pixelsignal, and in FIG. 7 e: a write pixel signal.

Reducing the duration of the column signal provides two benefits:

1) This variant minimizes the interfering signal since the pixels innon-selected rows receive the voltage C′ during time τ_(c) only, whichis close to τ₂/2, for example. 2) By shortening the column pulse whilesynchronizing its drop with that of the row pulse, the “slow” descent ofthe pixel signal takes place in three plateaus. With this method, whendeleting, the hydrodynamic flow of the liquid crystal is reducedcompared with that obtained with a pulse having two plateaus. Themaximum instantaneous voltage drop between each of the three plateaus issmaller than between two plateaus, for identical row voltage. Thistherefore encourages tilting towards the uniform texture U to a greaterextent. For writing, the hydrodynamic flow is not modified compared withthe two-plateau situation, since the maximum instantaneous voltage dropis identical. The inventors have shown that this method makes itpossible, without complicating the control electronics, to obtainswitching between the two states even when the viscosity of the liquidcrystal material increases at low temperature.

Still more precisely, the signals shown in FIG. 7 are as follows.

The row signal shown in FIG. 7 comprises a first sequence of duration τ₁and amplitude A1 followed by a second sequence of duration τ₂ (greaterthan τ₁) and of amplitude A2 (less than A1). The rising and fallingfronts of these two sequences are practically vertical.

The delete column signal shown in FIG. 7 b comprises a pulse of durationτ_(c) and amplitude C′ of the same polarity as the row signal shown inFIG. 7 a. The rising and falling fronts of this signal are practicallyvertical. The duration τ_(c) is less than the duration τ₂. The fallingfront of the delete column signal is synchronized on the falling frontof the row signal.

The write column signal shown in FIG. 7 c differs from the delete columnsignal shown in FIG. 7 b by a polarity reversal. Thus, in FIG. 7 c thereis a pulse of duration τ_(c) and amplitude C′, with vertical rising andfalling fronts, the falling front being synchronized with the fallingfront of the row signal.

As shown in FIG. 7 d, the voltage present across the terminals of thepixel when deleting comprises a run of three crenellations havingvertical rising and falling fronts. The first plateau is of amplitude A1and lasts for τ₁. The second plateau is amplitude A2 and lasts forτ₂−τ_(c). The third is plateau of amplitude A2−C′ and lasts for τ_(c).

As shown in FIG. 7 e, the voltage present across the terminals of thepixel when writing likewise comprises three successive plateaus withvertical rising and falling fronts: a first plateau of duration τ₁ andamplitude A1; a second plateau of amplitude A2 and duration τ₂−τ_(c);and a third plateau of amplitude A2+C′, of duration τ_(c).

Nevertheless, it should be observed that when deleting the pixel, theintermediate plateau has an amplitude A2 lying between the highestinitial amplitude A1 and the lowest final amplitude A2−C′, whereas whenwriting the pixel, the intermediate amplitude A2 is less than thehighest initial amplitude A1 and the final amplitude A2+C′.

EXAMPLE 2 Modifying the Waveform of the Column Pulse

The waveform of the column signal is modified so as to reduce its rmsvoltage compared with that of a standard column signal made up ofrectangular pulses. The duration of the column signal may also bereduced relative to the conventional τ₂, so as to benefit from theadvantages of variant 1.

Illustration 1

As a first example, use is made of a ramp type column signal. Theamplitude of this column signal increases linearly with time until itreaches a maximum peak voltage C″, and is then suddenly reduced to zerosynchronously with the end of the row pulse.

The maximum value of the column signal C″ can be increased relative tothe conventional value C, thus making it easier to switch between thetwo textures (cf. the electro-optical curve of FIG. 4).

An example of such signals is given in FIG. 8. In this case also, therecan be seen in FIG. 8 a: a row signal, in FIG. 8 b: a column deletesignal, in FIG. 8 c: a column write signal, in FIG. 8 d: a pixel deletesignal, and in FIG. 8 e: a pixel write signal. The column pulse is theduration τ_(c) and its waveform comprises a slope having a maximum C″.

Still more precisely, the signals shown in FIG. 8 are as follows.

The row signal shown in FIG. 8 a comprises a first sequence of durationτ₁ and amplitude A1 followed by a second sequence of duration τ₂(greater than τ₁) and an amplitude A2 (less than A1). The rising andfalling fronts of these two sequences are practically vertical.

The column delete signal shown in FIG. 8 b comprises a pulse of durationτ_(c) comprising a linear ramp rising front reaching the amplitude C″followed by a vertical falling front.

The column write signal shown in FIG. 8 c differs from the column deletesignal shown in FIG. 8 b by a polarity reversal. In FIG. 8 c, there canthus be seen a pulse of duration τ_(c) having a linear rising front thatreaches the amplitude C″ followed by a vertical falling front.

The voltage present across the terminals of the pixel when deleting, asshown in FIG. 8 d, comprises three successive sequences: a firstsequence of amplitude A1 and duration τ₁; a second sequence of amplitudeA2 and duration τ₂−τ_(c); and a third sequence of progressivelydecreasing amplitude of duration τ_(c) going from an initial amplitudeA2 to a final amplitude A2−C″.

In this case also, the value A2 in FIG. 8 d is an intermediate value.

The voltage present across the terminals of the pixel when writinglikewise comprises three successive sequences: a first sequence A1 ofamplitude A1 and duration τ₁; a second sequence of amplitude A2 andduration τ₂−τ_(c); and a third sequence of progressively increasingamplitude, of duration τ_(c), going from the initial value A2 to thehigher value A2+C″. As in the case of FIG. 8 c, and in a mannercomparable to FIG. 7 e, the value A2 is an intermediate value.

Illustration 2

By way of example, a column signal is used that increases through twoplateaus C1 and C2 of respective durations τ_(c1) and τ_(c2). An exampleof such signals is given in FIG. 9. Here again, there can be seen inFIG. 9 a: a row signal, in FIG. 9 b: a column delete signal, in FIG. 9c: a column write signal, in FIG. 9 d: a pixel delete signal, and inFIG. 9 e: a pixel write signal. The column pulse of durationτ_(c)=τ_(c1)+τ_(c2) and its waveform comprises two plateaus.

Multiplexing Variants—Obtaining a Mean Value of Zero

In order to take account of problems whereby certain liquid crystalmaterials degrade by electrolysis on being subjected to a direct current(DC) voltage, it is often useful to apply signals to the pixels of zeromean value. FIGS. 10, 11, and 12 show techniques enabling thetheoretical signals of FIG. 6 to be transformed into symmetrical signalsof zero mean value.

In FIG. 10, referred to as “row symmetrization”, identical signals ofopposite polarity follow one another to form the row selection signal.FIGS. 10 a, 10 b, 10 c, 10 d, and 10 e show respectively row signals,column delete signals, column write signals, delete signals across theterminals of a pixel, and write signals across the terminals of a pixel.Row symmetrization may be total, i.e. applied both to row signals and tocolumn signals, as shown in FIG. 10, or it may be partial, i.e. it maybe applied to row signals only and not to column signals. In which case,the column signal for selecting texture can be conserved.

Another symmetrization technique is shown in FIG. 11 which is referredto as “frame symmetrization”. The signals are the same as in FIG. 6, buttheir signs are reversed on each change of image. In this case also,symmetrization can be partial or total.

In the above circumstances and because of the symmetrization, the rowdriver signal needs to deliver a voltage of ±A1, i.e. a total excursionof 2·A1. A considerable simplification of the drivers can be obtained byreducing the excursion maximum to a value of less than 2·A1. To do this,it suffices to change synchronously the operating midpoint V_(M) of therow signal and of the corresponding column signal during the secondpolarity. Starting from the circumstances shown in FIG. 10, thisconsists in adding a common voltage V_(M) to all of the row and columnsignals during the symmetrization stage. FIG. 12 gives an example of asignal V_(M)=0 during the first polarity and V_(M) other than 0 duringthe second polarity. This principle is applicable with V_(M) differentfrom zero during the first polarity followed by V_(M) different fromzero during the second polarity. The important point is that the voltageacross the terminals of the pixel remain unchanged, as shown in FIG. 10.In this case also, FIGS. 12 a, 12 b, 12 c, 12 d, and 12 e representrespectively row signals, column delete signals, column write signals,delete signals across the terminals of a pixel, and write signals acrossthe terminals of a pixel.

All of these symmetrization means can be applied to the above-describedcolumn signals.

Limitation of the Conventional BiNem Multiplexing Method in Terms ofSpeed

When addressing a single row at a time using one of the above-describedmethods, the minimum time interval between addressing two rows is equalto τ₁+τ₂ or to 2(τ₁+τ₂) if the polarities are alternated duringaddressing of a given row (cf. FIG. 10). For example, the followingvalues can be used: τ₁=1 ms, τ₂=1 ms, and τ_(c)=200 μs, giving a minimumrow addressing time of 2 μs if the polarity reversal takes place on aper frame basis (referred to as circumstance 1) and of 4 ms if polarityreversal takes place during row addressing (referred to as circumstance2).

Unfortunately, the duration which determines the state of a pixel(written or deleted) is shorter than said duration, and equal to τ_(c),i.e. 200 μs in the example described.

The time for addressing an image having 160 rows is thus at least 320ms, whereas the time needed for determining the state of all of thepixels is 200 μs×160=32 ms.

SUMMARY OF THE INVENTION

A general object of the present invention is to improve the bistabledisplay devices described in French Patent No. 96/04447. Such devicesare generally referred to as “BiNems”. This terminology is used in thepresent patent application. The structure of such devices is describedin greater detail below.

Still more precisely, the object of the invention is to reduce the timerequired to address an image displayed on a screen of the multiplexedBiNem type.

As mentioned above, the present invention applies in particular toso-called BiNem devices using two textures, one that is uniform orlightly twisted and referred to as U, in which the molecules are atleast substantially parallel to one another, and the other being twistedand referred to as T which differs from the first by a twist of theorder of ±180°.

The inventors propose a novel method of addressing a multiplexed BiNemscreen enabling an image to be displayed more quickly by addressing aplurality of rows simultaneously with time overlap between row pulses.

To this end, the inventors provide a method of electrically addressing amatrix screen of bistable nematic liquid crystals with breaking ofanchoring, the method comprising the steps which consist in applyingcontrolled electrical signals respectively to row electrodes and tocolumn electrodes of the screen, and being characterized in that itfurther comprises the steps which consist in simultaneously addressing aplurality of rows using similar row signals that are offset in time by aduration greater than or equal to the time column voltages, said rowaddressing signals comprising in a first period at least one voltagevalue serving to break the anchoring of all of the pixels in the row,followed by a second period enabling the final states of the pixelsmaking up the address row to be determined, said final states being afunction of the value of each of the electrical signals applied to thecorresponding columns.

The present invention also provides a device for addressing a matrixscreen.

BRIEF DESCRIPTION OF THE FIGURES

FIG. 1 shows the principle of a conventional multiplexed matrix screen.The active zone of one pixel is situated at the intersection between rowand column electrodes. Arbitrarily, the row electrodes are shown asbeing on the top substrate or plate, while the column electrodes areshown as being on the bottom substrate or plate. When row N isaddressed, the column signals are applied simultaneously to all of thecolumns, after which addressing moves onto the following row.

FIG. 2 is a diagram showing the state of the art corresponding to FrenchPatent No. 96/04447 and more precisely showing one pixel of the liquidcrystal cell, and in this pixel, the two textures that are stablewithout any field being applied to the molecules: the textures beingreferred to as uniform U and twisted T. The central portion of thefigure shows the texture of the molecules with a field applied betweenthe electrodes carried by the two substrates. The arrows show therotations of the molecules when the field ceases.

FIG. 3 shows the conventional pixel signals enabling switching betweenthe two textures. The drop time of the write signal lies in the range afew microseconds and a few tens of microseconds. Two delete signals areproposed: one is a pulse followed by a ramp whose drop time is greaterthan three times the duration of the pulse, the other is a downwardstaircase, the signal having two plateaus.

FIG. 4 shows an example of an electro-optical curve for a liquid crystalpixel operating in application of the conventional principle shown inFIG. 2. The first applied voltage plateau is equal to 16 V, and thedegree of optical transmission is a function of the value of the secondplateau. There can be seen two operating points that are compatible withmultiplexed addressing.

FIG. 5 shows the correspondence between the value of the second plateauand the texture obtained in a conventional device. In the example ofFIG. 4, the uniform texture U is obtained for a second plateau having avalue lying in the range 5 V to 9 V. For a second plateau having a valuelying in the range 0 to 5 V, or in the range 9 V to 16 V, a twistedtexture T is obtained.

FIG. 6 shows the row and column signals for conventional multiplexedaddressing: obtaining one or the other of the two textures as a functionof the sign of the column signal.

FIG. 7 shows a variant of a novel signal waveform proposed in FrenchPatent No. 02/01448. The column pulse lasts for a time that is shorterthan the duration of the second plateau of the row signal and presents acrenellated waveform with a drop that is synchronized with the drop ofthe second plateau of the row signal.

FIG. 8 shows another variant of a novel signal waveform described inFrench Patent No. 02/01448. The column pulse lasts for a length of timethat is shorter than the duration of the second row signal plateau andpresents a ramp waveform with a drop that is synchronized with the dropof the second plateau of the row signal.

FIG. 9 shows yet another variant of a novel signal waveform described inFrench Patent No. 02/01448. The column pulse lasts for a length of timethat is shorter than the duration of the second plateau of the rowsignal, and it has a two-plateau waveform with a drop that issynchronized with the drop of the second plateau of the row signal.

FIG. 10 shows a conventional improvement proposed to avoid polarizing aliquid crystal cell, since that might lead to slow degradation of thematerial by electrolysis. The row and column signals are madesymmetrical, so that their mean value becomes zero.

FIG. 11 shows another conventional version in which symmetry is obtainedby reversing polarity from one image to the next.

FIG. 12 shows signals enabling symmetrical signals to be applied to thepixels while minimizing the voltage excursion of the control circuits.Under such circumstances, non-selected rows receive a row signal equalto the mean of the column signal instead of receiving no signal as underthe circumstances described above.

FIG. 13 shows the principle of addressing a BiNem screen by time overlapof the addressing pulses for consecutive rows (in this figure sevenconsecutive rows) without symmetrization.

FIG. 14 shows the principle of addressing a BiNem screen by time overlapof the addressing pulses for consecutive rows (in this figure threeconsecutive rows) with frame symmetrization.

FIG. 15 shows the principle of addressing a BiNem screen by time overlapof the addressing pulses for consecutive rows (in this figure threeconsecutive rows) with both row and frame symmetrization.

FIG. 16 shows the principle of addressing a BiNem screen by time overlapof the addressing pulses for consecutive rows (in this figure threeconsecutive rows) with total row symmetrization.

FIG. 17 shows the principle of addressing a BiNem screen by time overlapof the addressing pulses for consecutive rows (in this figure threeconsecutive rows) with partial row symmetrization.

FIG. 18 shows the principle of addressing a BiNem screen by time overlapof the addressing pulses for non-consecutive rows.

FIG. 19 shows the principle of addressing a BiNem screen by time overlapof the addressing pulses for consecutive rows, with a two-plateau rowsignal and a column signal with square waveform.

FIG. 20 shows an example of a row pulse waveform for addressing a BiNemscreen with time overlap of addressing pulses for rows using athree-plateau row signal during stage C for breaking anchoring.

FIG. 21 shows an example of a row pulse waveform for addressing a BiNemscreen by time overlap of row addressing pulses using a row signalhaving five plateaus during stage C of breaking anchoring.

DETAILED DESCRIPTION OF THE INVENTION

Because of the specific nature of the BiNem screen, in which switchingtakes place and is perceived only at the end of the application ofsignals to the terminals of the pixel, the constraints on implementingaddressing for a plurality of rows simultaneously are very differentfrom those that apply to a conventional LCD screen that obeys the Altand Plesko criterion. In a conventional LCD screen subject to the Altand Plesko criterion, the voltage applied at each instant contributes tothe optical state that is obtained at the pixel insofar as it has aneffect on the mean rms voltage that is applied thereto. For a BiNem typeLCD, it is only the waveform at the end of the pulse applied to thepixel that influences switching between the two textures, and thus thatinfluences the final optical state. It is therefore possible to proposean addressing scheme in which time overlap exists between a plurality ofrows.

The time offset between the rows is no longer equal to the durationτ_(L) as described in document [1], and its value is τ_(D) where:τ_(c)≦τ_(D)<τ_(L)where τ_(L) is the row addressing time which comprises at least twoaddressing stages (stage C for breaking anchoring and stage S forselecting texture) and τ_(c) is the duration of the column signal.

The present invention offers numerous advantages over the prior art.Three main advantages are described below.

First Advantage of the Invention: Speed of Addressing the Image

Let x be the number of rows that are addressed simultaneously.

For a given value of τ_(L), the optimum number of rows that can beaddressed simultaneously while taking advantage of a saving in time is:

with no symmetrization or with “frame” symmetrization:x _(opt)=integer portion of [τ_(L)/τ_(D)]

for “row” symmetrization:x _(opt)=integer portion of [2·τ_(L)/τ_(D)]A worked example: τ_(L)=2 ms; τ_(D)=200 μs, giving:

no symmetrization or “frame” symmetrization:x_(opt)=10

“row” symmetrization, whether partial or total:x_(opt)=20

The time required to address the x rows in accordance with the inventionis τ_(L)+[τ_(D)·(x−1)] which should be compared with x·τ_(L) thatapplies to standard sequential addressing.

The saving in addressing time over an image is calculated as follows:

Let T1 be the addressing time of an n-row image using the standardmethod of one row at a time, and let Tx be the time for addressing the nrows in accordance with the invention (x rows at a time). The followingrelationship applies:Tx≈T1/x for a large number n of rows.

A worked example with no symmetrization or with frame symmetrization:τ_(L)=1.2 msτ_(c)=100 μs and τ_(D)=200 μs

By addressing three rows at a time (x=3), the conventional method wouldtake 3.6 ms for those three rows while the addressing method of theinvention performs said addressing in 1.6 ms.

For an image having 160 rows:T1=160×1.2=190 msT3=(160/3)×1.6=85 ms

The time for addressing the image has been reduced by a factor of morethan 2.

Second Advantage of the Invention: Improving Switching and Reducing RowVoltages

Because of the time overlap, it is possible to increase the duration ofstage C without reducing the display rate. This increase makes itpossible to reduce the breaking voltages down to a limit value close tothe static breaking threshold. Under such circumstances, the adjustmentexcursion of the row and column voltages needed to guarantee goodoperation is considerably reduced. For example, the screen operates overa temperature range of more than 10° C. without requiring these voltagesto be adjusted, which is not true for fast operation without timeoverlap. To obtain maximum benefit from this advantage, the number ofrows addressed simultaneously may be selected to be greater thanx_(opt). The time saving will remain that corresponds to x_(opt), butthe same row can continue to be addressed for longer.

In addition, the reduction in the breaking voltage makes it possible touse drivers operating at lower voltage and that are therefore cheaper.

Third Advantage: Simplicity of the Row Signals

In the addressing system using time overlap, it is clear that aplurality of rows are addressed simultaneously. Nevertheless, the rowselection signals can remain very simple, and there is no need tosatisfy a condition of orthogonality, not even approximately, unlike thesignals that are needed for implementing MLA.

Furthermore, the present invention can give rise to numerous variantimplementations. Two main variants are described in succession belowcomprising respectively: 1) addressing a plurality of consecutive rowswith offset; and 2) addressing a plurality of non-consecutive rows, withoffset.

Variant 1: Offsetting a Plurality of Consecutive Rows

An example of a timing diagram corresponding to x=7 consecutive rowsaddressed simultaneously with a time offset τ_(D) from one row to thenext is shown in FIG. 13. The column signals corresponding to each roware sent sequentially once every τ_(D).

The row signal has a total duration τ_(L)=xτ_(D), which in this casegives τ_(L)=7τ_(D), for a column signal of duration τ_(c).

In FIG. 13 (as in FIGS. 14 to 18), the shaded blocks correspond to timesduring which the rows and the columns are addressed, without specifyingthe waveforms of the applied pulses. This figure shows the generalprinciple of time overlap for row pulses, which principle is independentof the content of the blocks, which corresponds to the waveform of therow and column pulses.

On examining FIG. 13, it will be understood that the beginning of therow signal for the (i+x)^(th) row is synchronized on the end of the rowsignal for the i^(th) row, i.e. in this case the beginning of the rowsignal for the eighth row is synchronized on the signal for the firstrow.

FIG. 13 is a diagram showing the principle of time overlap for rowpulses when there is no symmetrization.

The bottom of FIG. 13 (and also of FIGS. 14 to 19) shows firstly anexample of the time position of the column signal with τ_(c)=τ_(D), andsecondly an example of the time position of the column signal withτ_(c)<τ_(D). In both circumstances, the end of the column signal ofduration τ_(c) is synchronized with the end of the corresponding rowsignal of duration τ_(L).

FIG. 14 is a diagram showing the principle of time overlap with framesymmetrization.

In this circumstance, the polarities of the row signals and of thecolumn signals are reversed from one image p to the following image p+1.

The column signals corresponding to each row are sent sequentially everyτ_(D), which value corresponds to the time offset between two successiverow signals of the simultaneous addressing.

FIG. 15 is a diagram showing the principle of time overlap for framesymmetrization, with alternating sign for the row pulse.

In this case, firstly the polarities of the row signal and of the columnsignal are reversed from image p to the following image p+1. Secondly,the polarities of two successive row signals, and also of two successivecolumn signals are also reversed.

FIG. 16 is a diagram showing the principle of total row symmetrization.

In this case, each row signal comprises two successive adjacentsequences of equal duration, presenting respective opposite polarities,and the column signal is split into two sequences whose ends aresynchronized with the ends respectively of the first sequence and of thesecond sequence of the associated row signal, the polarities of the twocolumn signal sequences likewise being reversed.

FIG. 17 is a diagram showing the principle for partial rowsymmetrization.

Under such circumstances, each row signal comprises two successiveadjacent sequences of equal duration, presenting respective oppositepolarities, and the end of the column signal is synchronized on the endof the second associated row signal sequence.

In general, all of the above-mentioned variations of symmetrization,whether they apply to frame symmetrization or to row symmetrization, canrelate either to symmetrizing row signals and column signals, or tosymmetrizing row signals alone.

Drawback of Variant 1: Limit on the Number of Consecutive Rows that canbe Addressed Simultaneously

When a row is addressed, during the addressing time, nearly all of themolecules are tilted into the homeotropic state, and the lighttransmission of the row is disturbed. When addressing one row at a time,if the size of the row is smaller than the resolution of the eye, thenthe observer will not be inconvenienced. However, if a plurality ofconsecutive rows are addressed and therefore disturbed, a larger zonewill be optically disturbed and will become visible and thus disturbingfor an observer.

Variant 2: Offsetting a Plurality of Non-Consecutive Rows

In order to overcome the visible disturbance due to disturbing aplurality of consecutive rows simultaneously (a traveling bar of sizemuch greater than the width of one row), it can be advantageous to spaceapart the rows which are addressed with time overlap.

The timing diagram of FIG. 18 shows this mode of addressing in anaddressing example that possesses a time offset of one-third of the rowsignal duration τ_(L): τ_(L)=3τ_(D). In this example, the maximum numberof rows that can be addressed simultaneously is equal to three.

The same symmetrization options as for consecutive rows can be selected.

More precisely, and in general, in the context of the present invention,it is possible to make provision for addressing simultaneously i moduloj rows, i.e. rows i, i+j, i+2j, etc., by providing a row signal ofduration τ_(L)=jτ_(D), with a time offset τ_(D) between two successiverow signals applied simultaneously and with an offset τ_(L) betweensuccessive blocks of row signals applied simultaneously.

The row signals and the column signals corresponding to the blocks areshaded in FIGS. 13 to 18 and can be implemented in a wide variety ofways.

Some of them are described below in non-limiting manner.

The row and column pulses may in particular comply with the waveformsdescribed below.

During the anchoring breaking stage C, voltage is applied to the rowsignal only.

The duration of the selection stage S is equal to the duration of thecolumn pulse.

Column Pulses

The waveform of these pulses may correspond to each of the examplesdescribed in the prior art or to a combination of these examples:

column signal duration less than or equal to the duration of the lastplateau of the row signal;

column signal of arbitrary waveform: square, ramp, staircase, etc.;

column signal of duration τ_(c) equal to τ_(D);

column signal of duration τ_(c) less than τ_(D).

Row Pulse

Two-plateau row signal

FIG. 19 shows an example of BiNem screen addressing with time overlap ofrow addressing pulses using Variant 1 (consecutive rows) with atwo-plateau row signal and a square waveform column signal of durationshorter than the second plateau of the row signal.

Multi-plateau row signal during stage C, with at least one voltageenabling anchoring to be broken (A1 as defined in the prior art). Thevoltage level of stage C is equal to A2 as defined in the prior art. Athree-plateau example is given in FIG. 20 and a five-plateau example isgiven in FIG. 21.

In these two examples, the row driver need generate only two voltagelevels: a non-selection level and a selection level modulatedalternately between A1 and A2. This corresponds to the simplest possiblestructure for a row driver. Naturally, it is possible to devisesolutions using a row driver that is capable of generating a largernumber of voltage levels. The row signal can then have a waveform thatis more complex, but it must nevertheless comply with the constraintsfor breaking anchoring (stage C) and for selecting texture (stage S).

A multi-plateau row signal during stage S, with at least one drop at theend enabling texture to be selected.

Naturally, the present invention is not limited to the particularembodiments described above, but extends to all variants within itsspirit.

1. A method of electrically addressing a matrix screen of bistable nematic liquid crystals with breaking of anchoring, the method comprising applying controlled electrical signals respectively to row electrodes and to column electrodes of the screen, and further comprising simultaneously addressing a plurality of rows using similar row signals that are offset in time by a duration greater than or equal to the time column voltages, said row addressing signals comprising in a first period at least one voltage value serving to break the anchoring of all of the pixels in the row, followed by a second period enabling the final states of the pixels making up the address row to be determined, said final states being a function of the value of each of the electrical signals applied to the corresponding columns.
 2. A method of addressing a matrix screen of bistable nematic liquid crystals with breaking of anchoring according to claim 1, wherein the screen uses two textures, one texture being uniform or lightly twisted in which the molecules are at least substantially parallel to one another, and the other texture differing from the first by a twist of the order of ±180°.
 3. A method according to claim 1, wherein the ends of the column signals are synchronized with the ends of the row signals.
 4. A method according to claim 1, wherein τ_(c)≦τ_(D)<τ_(L) in which relationship: τ_(D) represents the time offset between two row signals; τ_(L) represents the row addressing time comprising at least an anchoring breaking stage and a texture selection stage; and τ_(c) represents the duration of a column signal.
 5. A method according to claim 1, wherein the time for addressing x simultaneously addressed rows is equal to τ_(L)+[τ_(D)(x−1)] in which relationship: τ_(D) represents the time offset between two row signals; and τ_(L) represents the row addressing time including at least an anchoring breaking stage and a texture selection stage.
 6. A method according to claim 1, wherein the rows addressed simultaneously in time overlap are adjacent rows.
 7. A method according to claim 1, wherein the rows addressed simultaneously in time overlap are rows that are spaced apart.
 8. A method according to claim 7, further comprising simultaneously addressing i modulo j rows, i.e. rows i, i+j, i+2j, etc., by providing a row signal of duration τ_(L)=jτ_(D), by offsetting two successive simultaneously applied row signals in time by τ_(D), and by offsetting the successive blocks of simultaneously applied row signals by τ_(L).
 9. A method according to claim 1, wherein parameters of the signals applied to the screen column electrodes are adapted to reduce the rms voltage of interfering pixel pulses in order to reduce the interfering optical effects of the addressing.
 10. A method according to claim 1, wherein parameters of the signals applied to the screen column electrodes are adapted to reduce the rms voltage of the interfering pixel pulses to a value of less than the Freedericksz voltage, so as to reduce the interfering optical effects of the addressing.
 11. A method according to claim 10, wherein the parameters adapted to the electrical signal are selected from the group consisting of the waveform, the duration, and the amplitude of the column signal.
 12. A method according to claim 1, wherein a duration of the column signal is less than the duration of a last plateau of the row pulse.
 13. A method according to claim 1, wherein the column signal presents a squarewave shape.
 14. A method according to claim 1, wherein the column signal presents a ramp shape.
 15. A method according to claim 1, wherein x consecutive rows are addressed simultaneously with a time offset τ_(D) from one row to the next, the column signals corresponding to each row being sent sequentially once every τ_(D), and each row signal having a total duration of not less than τ_(L)=xτ_(D).
 16. A method according to claim 1, wherein a beginning of the row signal for the (i+x)^(th), row is synchronized with an end of the row signal for the i^(th) row.
 17. A method according to claim 1, wherein the row signals do not present any symmetrization.
 18. A method according to claim 1, wherein the signals present frame symmetrization.
 19. A method according to claim 18, wherein polarities of the row signals are reversed from one image p to the following image p+1.
 20. A method according to claim 18, wherein polarities of the row signals and polarities of the column signals are reversed from one image p to the following image p+1.
 21. A method according to claim 18, wherein polarities of two successive row signals are reversed.
 22. A method according to claim 18, wherein polarities of two successive row signals, and also of two successive column signals are reversed.
 23. A method according to claim 17, wherein the number of rows addressed simultaneously is not less than: x _(opt)=integer portion [τ_(L)/τ_(D)] in which relationship: τ_(D) represents the time offset between row signals; and τ_(L) represents the row addressing time comprising at least an anchoring breaking stage and a texture selection stage.
 24. A method according to claim 1, wherein the signals present row symmetrization.
 25. A method according to claim 24, wherein each row signal comprises two successive adjacent sequences presenting respective opposite polarities.
 26. A method according to claim 24, wherein the column signal is split into two sequences whose ends are synchronized respectively with the end of the first sequence and with the end of the second sequence of the associated row signal, polarities of the two column signal sequences being likewise reversed.
 27. A method according to claim 24, wherein the end of the column signal is synchronized with the end of the second sequence of the associated row signal.
 28. A method according to claim 24, wherein the polarities of two successive row signals are reversed.
 29. A method according to claim 24, wherein the polarities of two successive row signals and also of two successive column signals are reversed.
 30. A method according to claim 24, wherein the number of rows addressed simultaneously is not less than: x _(opt)=integer portion [2τ_(L)/τ_(D)] in which relationship: τ_(D) represents the time offset between two row signals; and τ_(L) represents the row addressing time comprising at least an anchoring breaking stage and a texture selection stage.
 31. A method according to claim 1, wherein the column signal is selected from the group comprising: a column signal of duration less than or equal to the duration of the last plateau of the row signal; a column signal of duration τ_(c) equal to τ_(D); and a column signal of duration τ_(c) less than τ_(D), where τ_(D) represents the time offset between two row signals, while τ_(c) represents the duration of a column signal.
 32. A method according to claim 1, wherein the row signal is a two-plateau signal: a plateau during the anchoring breaking stage; and a plateau during a texture selection stage.
 33. A method according to claim 1, wherein the row signal is a multi-plateau signal during the anchoring breaking stage.
 34. A method according to claim 1, wherein the row signal is a multi-plateau signal during a texture selection stage.
 35. A device for electrically addressing a matrix screen having a bistable nematic liquid crystal with breaking of anchoring, the device comprising means suitable for applying controlled electrical signals respectively to the row electrodes and to the column electrodes of the screen, and further comprising the means suitable for simultaneously addressing a plurality of rows using similar row signals that are offset in time by a duration greater than or equal to the time column voltages are applied, said row addressing signals comprising in a first period at least one voltage value serving to break the anchoring of all of the pixels in the row, followed by a second period enabling the final states of the pixels making up the address row to be determined, said final states being a function of the value of each of the electrical signals applied to the corresponding columns.
 36. A device for addressing a matrix screen of bistable nematic liquid crystals with breaking of anchoring according to claim 35, wherein the screen uses two textures, one texture being uniform or lightly twisted in which the molecules are at least substantially parallel to one another, and the other texture differing from the first by a twist of the order of +180°.
 37. A device according to claim 35, wherein the ends of the column signals are synchronized with the ends of the row signals.
 38. A device according to claim 35, wherein τ_(c)≦τ_(D)<τ_(L) in which relationship: τ_(D) represents the time offset between two row signals; τ_(L) represents the row addressing time comprising at least an anchoring breaking stage and a texture selection stage; and τ_(c) represents the duration of a column signal.
 39. A device according to claim 35, wherein the time for addressing x simultaneously addressed rows is equal to τ_(L)+[τ_(D)(x−1)] in which relationship: τ_(D) represents the time offset between two row signals, and τ_(L) represents the row addressing time including at least an anchoring breaking stage and a texture selection stage.
 40. A device according to claim 35, wherein the rows addressed simultaneously in time overlap are adjacent rows.
 41. A device according to claim 35, wherein the rows addressed simultaneously in time overlap are rows that are spaced apart.
 42. A device according to claim 41, further including means for simultaneously addressing i modulo j rows, i.e. rows i, i+j, i+2j, etc., by providing a row signal of duration τ_(L)=jτ_(D), by offsetting two successive simultaneously applied row signals in time by τ_(D), and by offsetting the successive blocks of simultaneously applied row signals by τ_(L).
 43. A device according to claim 35, wherein parameters of the signals applied to the screen column electrodes are adapted to reduce the rms voltage of interfering pixel pulses in order to reduce interfering optical effects of the addressing.
 44. A device according to claim 35, wherein parameters of the signals applied to the screen column electrodes are adapted to reduce the rms voltage of the interfering pixel pulses to a value of less than, the Freedericksz voltage, so as to reduce interfering optical effects of the addressing.
 45. A device according to claim 44, wherein the parameters adapted to the electrical signal are selected from the group consisting of: the waveform, the duration, and the amplitude of the column signal.
 46. A device according to claim 35, wherein a duration of the column signal is less than a duration of a last plateau of the row pulse.
 47. A device according to claim 35, wherein the column signal presents a squarewave shape.
 48. A device according to claim 35, wherein the column signal presents a ramp shape.
 49. A device according to claim 35, wherein x consecutive rows are addressed simultaneously with a time offset τ_(D) from one row to the next, the column signals corresponding to each row being sent sequentially once every τ_(D), and each row signal having a total duration of not less than τ_(L)=xτ_(D).
 50. A device according to claim 35, wherein a beginning of the row signal for the (i+x)^(th) row is synchronized with an end of the row signal for the i^(th) row.
 51. A device according to claim 35, wherein the row signals do not present any symmetrization.
 52. A device according to claim 35, wherein the signals present frame symmetrization.
 53. A device according to claim 52, wherein polarities of the row signals are reversed from one image p to the following image p+1.
 54. A device according to claim 52, wherein the polarities of the row signals and polarities of the column signals are reversed from one image p to the following image p+1.
 55. A device according to claim 52, wherein polarities of two successive row signals are reversed.
 56. A device according to claim 52, wherein the polarities of two successive row signals, and also of two successive column signals are reversed.
 57. A device according to claim 51, wherein the number of rows addressed simultaneously is not less than: x _(opt)=integer portion [τ_(L)/τ_(D)] in which relationship: τ_(D) represents the time offset between row signals; and τ_(L) represents the row addressing time comprising at least an anchoring breaking stage and a texture selection stage.
 58. A device according to claim 35, wherein the signals present row symmetrization.
 59. A device according to claim 58, wherein each row signal comprises two successive adjacent sequences presenting respective opposite polarities.
 60. A device according to claim 58, wherein the column signal is split into two sequences whose ends are synchronized respectively with the end of the first sequence and with the end of the second sequence of the associated row signal, polarities of the two column signal sequences being likewise reversed.
 61. A device according to claim 58, wherein an end of the column signal is synchronized on an end of the second sequence of the associated row signal.
 62. A device according to claim 58, wherein polarities of two successive row signals are reversed.
 63. A device according to claim 58, wherein polarities of two successive row signals and also of two successive column signals are reversed.
 64. A device according to claim 58, wherein the number of rows addressed simultaneously is not less than: x _(opt)=integer portion [2τ_(L)/τ_(D)] in which relationship: τ_(D) represents the time offset between two row signals; and τ_(L) represents the row addressing time comprising at least an anchoring breaking stage and a texture selection stage.
 65. A device according to claim 35, wherein the column signal is selected from the group comprising: a column signal of duration less than or equal to the duration of the last plateau of the row signal; a column signal of duration τ_(c) equal to τ_(D); and a column signal of duration τ_(c) less than τ_(D), where τ_(D) represents the time offset between two row signals, while τ_(c) represents the duration of a column signal.
 66. A device according to claim 35, wherein the row signal is a two-plateau signal: a plateau during the anchoring breaking stage and a plateau during a texture selection stage.
 67. A device according to claim 35, wherein the row signal is a multi-plateau signal during the anchoring breaking stage.
 68. A device according to claim 35, wherein the row signal is a multi-plateau signal during the texture selection stage. 